Dip direction
What are Dip and Dip direction
30/11/08 19:53 Filed in: Geomechanics
One of the keys to understand rock mechanics is to
understand the joint patterns within the rock.
Critically important in understand the joint
pattern is to understand the orientation of the
joints. To do this we use Dip, Dip Direction and
the related measurements.
The
first concept to understand is the dip of a
joint. We shall initially consider a single
planar joint within the rock such as a shear
or fault. To define the direction of the joint
in three dimensional space we typically use
two values. By doing this we can use the
conventional 360 degrees that is commonly used
to define angles and directions. The two
angles typically used are called Dip and Dip
Direction.
To consider Dip and Dip Direction we need to consider a line on the plane of the joint. Imagine a grid of lines on this plane, with one set of lines running across the plane at the same height, effectively forming contours not rising or falling across the plane. The second set of lines on this grid will run perpendicular to the contours. This second set of lines would be the steepest line that could be formed on the plane. This line is the key line from which we measure Dip and Dip Direction.
The Dip Direction can be determined by viewing the plane and the line we have drawn on plan. The orientation of the line can be specified using the 360 degrees of a compass bearing. At this point it is worth noting that this value is not absolute, but that the baseline needs to be specified. Normally the baseline orientation is North but this could be true north, grid north or magnetic north.
Having specified the direction of the line, we need to specify how steep the line is. We do this using the Dip of the joint. To obtain the dip we have to cut the plane of the joint along the Dip Direction of the joint. Having cut the joint plane the Dip of the joint can be measured by measuring the angle from the horizontal of the plan of the joint. This means that a steeper dip gives a steeper joint up to a maximum of 90 degrees. Typically dip is only measured as a positive angle between 0 and 90 degrees.
Dip
and Dip Direction are the typical values that
are quoted to give the joint orientation.
There are however a few other dimensions that
are used to specify the orientation of
features. Strike is a value used instead of
Dip Direction. The Strike is a line at 90
degrees to the Dip Direction. Referring back
to our original diagram the Strike follows the
contour lines running parallel to the slope.
Strike and Dip Direction are therefore
interrelated. It is important to note that
Strike may not be defined using the
conventional 360 degree compass, but may be
defined as being an angle East or West of
North such as N72E, 72 degrees East of North.
Strike can be represented on a plan by a long
line following the direction of the strike and
a short bar at 90 degrees showing the
direction of the dip.
Another method of defining Dip and Dip Direction is Trend and Plunge. Trend and plunge tend to be used with linear features such as tunnels, shafts or roads. Trend tends to follow the centreline of these features such as the axis of a tunnel and is defined in the same way as Dip Direction. The plunge is then defined in the same way as the dip so the steeper the plunge the steeper the slope of the tunnel.
It is important to understand Dip and Dip Direction when designing rock structures. When using these measurements of the orientation of joints you must clarify how it has been defined by the geologist to ensure that you are applying the measurements in the same manner as they have been defined.
The
first concept to understand is the dip of a
joint. We shall initially consider a single
planar joint within the rock such as a shear
or fault. To define the direction of the joint
in three dimensional space we typically use
two values. By doing this we can use the
conventional 360 degrees that is commonly used
to define angles and directions. The two
angles typically used are called Dip and Dip
Direction.
To consider Dip and Dip Direction we need to consider a line on the plane of the joint. Imagine a grid of lines on this plane, with one set of lines running across the plane at the same height, effectively forming contours not rising or falling across the plane. The second set of lines on this grid will run perpendicular to the contours. This second set of lines would be the steepest line that could be formed on the plane. This line is the key line from which we measure Dip and Dip Direction.
The Dip Direction can be determined by viewing the plane and the line we have drawn on plan. The orientation of the line can be specified using the 360 degrees of a compass bearing. At this point it is worth noting that this value is not absolute, but that the baseline needs to be specified. Normally the baseline orientation is North but this could be true north, grid north or magnetic north.
Having specified the direction of the line, we need to specify how steep the line is. We do this using the Dip of the joint. To obtain the dip we have to cut the plane of the joint along the Dip Direction of the joint. Having cut the joint plane the Dip of the joint can be measured by measuring the angle from the horizontal of the plan of the joint. This means that a steeper dip gives a steeper joint up to a maximum of 90 degrees. Typically dip is only measured as a positive angle between 0 and 90 degrees.
Dip
and Dip Direction are the typical values that
are quoted to give the joint orientation.
There are however a few other dimensions that
are used to specify the orientation of
features. Strike is a value used instead of
Dip Direction. The Strike is a line at 90
degrees to the Dip Direction. Referring back
to our original diagram the Strike follows the
contour lines running parallel to the slope.
Strike and Dip Direction are therefore
interrelated. It is important to note that
Strike may not be defined using the
conventional 360 degree compass, but may be
defined as being an angle East or West of
North such as N72E, 72 degrees East of North.
Strike can be represented on a plan by a long
line following the direction of the strike and
a short bar at 90 degrees showing the
direction of the dip.
Another method of defining Dip and Dip Direction is Trend and Plunge. Trend and plunge tend to be used with linear features such as tunnels, shafts or roads. Trend tends to follow the centreline of these features such as the axis of a tunnel and is defined in the same way as Dip Direction. The plunge is then defined in the same way as the dip so the steeper the plunge the steeper the slope of the tunnel.
It is important to understand Dip and Dip Direction when designing rock structures. When using these measurements of the orientation of joints you must clarify how it has been defined by the geologist to ensure that you are applying the measurements in the same manner as they have been defined.